Field of the Invention
The invention relates to a method and an apparatus for testing a plurality of fuel assemblies of a nuclear reactor, in particular a boiling water reactor, for fuel rod leaks while the fuel assemblies are resting under water on a working base, in particular while they are still inside a core assembly in a reactor pressure vessel. The fuel rods are heated in order to drive radioactive fission products out of defective fuel rods, and the radioactivity of the fission products that have been driven out is recorded in samples that are extracted from the environment of the fuel assemblies. The corresponding apparatus includes a hood, the opening of which faces downward and can be positioned over at least one of the resting fuel assemblies. At least one extractor hood with an extraction device leads out of the hood, and there is a detector device for analyzing the radioactivity of the extracted fission products.
In nuclear reactors, spent fuel rods have to be replaced with new fuel rods at regular intervals, and the new fuel rods are then redistributed through the core together with reusable fuel assemblies. Generally, the old fuel assemblies are lifted out of their working base, i.e. for example a lower core grid in the reactor pressure vessel, and are initially placed on a different working base, for example a storage rack in a dedicated water basin, in order subsequently to be returned to the reactor pressure vessel and inserted at the new position in the reactor core. The fuel assemblies are under water throughout the entire operation for radiation protection reasons, and to be transported they are held in a height-adjustable manner on the mast of a displaceable fuel handling machine.
Light water-cooled reactors have to be shut down, and for cost reasons operating pauses of this nature should be kept as short as possible.
However, irradiated fuel assemblies can only be reused if the cladding tubes of the fuel rods do not have any leaks through which radioactive fission products formed in the fuel as a result of the nuclear fission during the prior reactor operation could escape and unacceptably contaminate the cooling water of the reactor. As well as visual inspections of the fuel assemblies and testing of individual fuel rods by ultrasound or eddy-current probes, what is known as xe2x80x9csippingxe2x80x9d is a conventional method of identifying the fuel assembly which contain a fuel rod with a leak. First, a pressure difference is generated between the internal pressure in the fuel rod and the external pressure in the surrounding water, in order to drive out the fission products formed in the fuel rod filling in the maximum possible quantities and then to analyze samples extracted from the environment of the fuel assembly. Detectors for radioactive radiation can be used for the analysis. Detectors of this type may, for example, be particularly sensitive to gaseous fission products, such as xenon 133 or krypton 85, in gaseous samples or to water-soluble fission products (e.g. iodine 131 or cesium 134) in water samples.
When testing for leaks in irradiated fuel assemblies, reliability of detection and speed are particularly important criteria.
For this purpose, xe2x80x9cmast sippingxe2x80x9d has been developed, in which the search for leaks is carried out while the fuel assemblies are hanging from the mast of the handling machine and are being transported between the two above-mentioned working bases. In the case of pressurized water handling machines, a fuel assembly, in order to laterally protect its fuel rods, is lifted into a centering bell, which is then introduced into the hollow mast of the handling machine. Since the fuel assembly is lifted several meters, the hydrostatic pressure in the surrounding water falls with respect to the internal pressure in the fuel rods, pressure equalization taking place at the leaks in fuel rods, which causes the radioactive fission products to be driven out of the defective fuel rod. Dry sipping is then possible, in which the escaping gas bubbles collect at the top of the centering bell and are extracted together with a purge gas which is introduced into the centering bell from below, displaces the cooling water and also entrains gaseous fission products which have been adsorbed on the outer surface of the fuel rods. The extracted gas can be analyzed on-line in a detector device with an electronic evaluation device, i.e. the radioactivity of the gaseous fission products which have been driven out is recorded while the fuel assembly is still hanging from the handling mast. It is possible to dispense with the introduction and extraction of the purge gas, in which case only water is extracted from the top of the fuel assemblies until virtually all the cooling water which was originally present in the fuel assembly or the centering bell has been exchanged (xe2x80x9cdry sippingxe2x80x9d). During the exchange of the water, it is also possible for gas bubbles that originally escaped to be dissolved or at least entrained by the flow of water and to be released again together with dissolved fission products by degassing the extracted water in a degassing device, in order for their radioactivity subsequently to be recorded in a detector device.
In the boiling-water reactor, the mast of the handling machine is simply a telescopic arm with a downwardly projecting gripper on which the fuel assembly is held outside the mast. In this case too, the above-mentioned mast sipping methods are possible if the gripper is disposed in a downwardly open hood that has been fitted over the top fitting of the fuel assembly. This is because boiling water fuel assemblies have a fuel assembly channel that laterally surrounds the fuel rods and during the sipping is responsible for the function of the centering bell in the hollow mast of the handling machine. Depending on the size of the core, the mast sipping requires 50 to 120 hours. Although the fuel assembly should if possible be tested while the fuel assemblies are being transported, the mast sipping requires additional time.
Another conventional way of saving time in boiling water reactors is to test a plurality of fuel assemblies simultaneously by a hood that is divided by side walls into individual cells for accommodating the individual fuel assemblies. The simultaneous testing of the fuel assemblies may take place as dry sipping. Although this only leads to a slight hydraulic pressure difference between the at-rest position in the core and the position in which the sipping is carried out, the expulsion of the fission gases is increased by the fact that so much gas is introduced into the hood which has been fitted over the fuel-assembly top fittings that the upper edge of the fuel assembly channel of each top fitting is positioned in a gas cushion which virtually suppresses the circulation of cooling water on the fuel rods. Therefore, the afterheat of the fuel heats the internal volume of the fuel rods and thermally generates a pressure difference that sufficiently reinforces the hydraulic pressure difference. The gas bubbles that escape through leaks and bubble upward combine with the gas cushion beneath the hood. After a predetermined heating time, the gas cushion can be extracted together with the collected gaseous fission products in order to be analyzed, for example in a laboratory, for the presence of typical fission products.
This dry sipping can also be carried out without the fuel assemblies having to be lifted so far out of the reactor core that their lower end would be accessible for the introduction of purge gas or without the fuel assemblies having to be moved in the reactor core at all, i.e. while they are on their standard working base (the lower core grid in the reactor pressure vessel or a storage rack). Although this eliminates the time required to raise the fuel assembly, and in particular the time required to reliably pick up all the fuel assemblies which are to be tested simultaneously, it is necessary, and this takes virtually the same amount of time, to position the extractor hood in a precise position with respect to the working base. A time saving could result if a large number of fuel assemblies at the same time could be tested individually (i.e. by use of in each case a dedicated device for heating and extractionxe2x80x94i.e. requiring a considerable outlay on equipment) and the hood only had to be repositioned a few times. However, in the boiling water core there are in each case four fuel assemblies in a square mesh of the core grid, and the tight spatial conditions within a mesh in practice do not allow side walls, which could be used to form a dedicated gas cushion for collection of the rising fission gases at the top of each fuel assembly, still to be introduced between the fuel assembly channels thereof. A further difficulty is that the fuel assemblies generally undergo different growth and distortion as a result of the reactor radiation, but the upper edge of the fuel assembly channel has to reach the gas cushion if fission products from a fuel assembly are not to enter the gas cushion of another fuel assembly, which would invalidate the test.
Therefore, dry sipping, in which the fuel assemblies rest on their working base, would at most be suitable as a preliminary test in which in each case the four fuel assemblies belonging to a core grid mesh are together tested for leaks from all the fuel rods. The fission gases escaping from one of the four fuel assemblies are diluted in the common gas cushion and the long extraction lines in such a manner that long measurement times, e.g. in an analysis laboratory for removed gas samples, are required. If this results in a significant increase in the radioactivity over the environment, it would then be necessary for the leak-tightness of each fuel assembly of this mesh subsequently to be tested individually in some other way.
In a process in which the fuel assemblies rest on the working base, one obstacle to the idea of extracting water samples from the fuel assemblies after the heating and then degassing them rather than gas samples from a gas cushion (i.e. instead of dry sipping), is that during the heating gaseous fission gases escape into the gas cushion and are therefore lost to analysis of the gases dissolved in the extracted water. However, it is possible to detect the solid or liquid fission products that are released into the water. For this purpose, water samples that have been removed individually from the fuel assemblies can be tested in a laboratory (off-line). In this way, all the fuel assemblies of a core can be tested within 30 to 50 hours.
To increase the measuring rates for a radioactive gas that is only produced in small quantities, the small quantities are often mixed with a non-radioactive carrier gas and are passed through a configuration of detectors a number of times in a circuit. As a result, the radioactivity is recorded a number of times, in order to acquire a statistically significant deviation from the normal radioactivity of the environment. A statistical significance of this type is particularly important when detecting fission products from defective fuel rods if the fuel assemblies with the fuel rods have already been tested in the water basin of a nuclear reactor, since in that case there is already a relatively high background of radioactivity.
It is accordingly an object of the invention to provide a method and an apparatus for testing nuclear reactor fuel assemblies that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, in which the leak testing of the fuel rods of individual fuel assemblies is completed within the shortest possible time and using processes which are as simple as possible. It has been developed in particular for boiling water reactors and is described for such reactors, although it can also be applied to other fuel assemblies surrounded by fuel assembly channels or fuel assemblies (e.g. of pressurized water reactors) if they are in an insulating container which prevents the coolant in the fuel assembly from escaping at the sides.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for testing whether fuel rods of fuel assemblies resting on a working base and under water, of a nuclear reactor are leaking. The method includes heating at least one first fuel assembly of a first division of fuel assemblies for driving radioactive fission products out of a defective fuel rod contained in the first fuel assembly. The first fuel assembly is continuously tested by extracting samples of water and continuously degassing the water removed from an area around the first fuel assembly even during the heating resulting in gas. A radioactivity of gaseous fission products released in the gas is continuously recorded. A fuel assembly belonging to a second division of fuel assemblies is heated only if the first fuel assembly belonging to the first division of fuel assemblies has been tested.
To achieve this, the invention makes use of the method in which the fuel assemblies rest on the working base and under water while their fuel rods are being tested for leaks. The fuel assemblies are heated in order for the radioactive fission products to be driven out of defective fuel rods, and the testing is carried out by recording the radioactivity of fission products which have been driven out in samples which have been extracted from the environment of the fuel assemblies. In the most simple embodiment of the invention, the fuel assemblies are tested in succession, in which case a fuel assembly is heated, and the water is continuously extracted from the fuel assembly as a sample even as early as during the heating and is continuously degassed. This leads to gas being continuously liberated as a result of the radioactivity of the gaseous fission products that have been liberated being continuously recorded. The method is controlled online, i.e. the measurement results are produced continuously even during the continuous extraction, and then the end of the test can also be established from these results, so that the testing of another fuel assembly can then be commenced.
At any rate, the fuel assemblies are located in water in which the gaseous fission products, which may have been in the water even before the heating and may originate from other fuel. assemblies, may also be dissolved. A radioactive xe2x80x9cbackgroundxe2x80x9d of this nature may interfere with the meaningfulness of the measurements, since in this case not only are the fission products which have been driven out of the fuel assemblies during the heating recorded from the extracted water, but also fission products from this xe2x80x9cbackgroundxe2x80x9d may be released during the degassing. According to the invention, it is advantageous if a carrier gas which if possible has no radioactivity or the radioactivity of which is virtually constant at least for all the fuel assemblies which are to be tested in succession (i.e. for example sucked-in ambient air or nitrogen or another gas which is held in compressed form in cylinders), is passed through the extracted water. In the water, the carrier gas forms bubbles that also take up the residual quantities of dissolved fission products that are not released simply by reducing the pressure or similar degassing measures. The water is then virtually completely degassed.
The radioactivity of the gases released can be recorded in a detector or in a configuration of a plurality of detectors connected in series, but it is advantageous for the fission products which have been released not to be passed through the detector device a number of times in a circuit. Although multiple detection of this nature increases the counting rates, it erroneously introduces a higher significance, since not only the fission products that have been driven out of the defective fuel rods but also the fission products of the background are recorded a number of times. It is then difficult to assess whether only released fission products from the background are being counted over and over again in the circuit, and the counting rates are increasing, or whether the rise is attributable to an increasing escape of fission products from heated, defective fuel rods. Rather, it is advantageous for the gas formed during the degassing of the water only to be passed through the detector device once and then to be passed into an exhaust duct or disposed of in some other way.
First, a particularly preferred embodiment of the invention will be explained in general terms but with reference to a specific example. In this case, the fuel assemblies of the core are divided into groups and divisions; the text in brackets in each case relates to the specific example in order to facilitate understanding.
The embodiment takes into account the fact that the fuel assemblies are located in meshes of the core grid thatxe2x80x94apart from individual meshes at the edgexe2x80x94contains four fuel assemblies in each mesh. The regular arrangement allows the fuel assemblies to be divided into clear groups, e.g. a first group of fuel assemblies, a second group and, if appropriate further groups, which each contain a first fuel assembly a second fuel assembly and, if appropriate further fuel assemblies. The fuel assemblies belonging to a first division that contains at least the first and second groups should be fully tested for leaks from their fuel rods before a first fuel assembly belonging to another division is tested.
Therefore, at least xe2x80x9cfirstxe2x80x9d fuel assemblies and xe2x80x9csecondxe2x80x9d fuel assemblies belong to the first division. According to this embodiment, the fuel assemblies belonging to the first group are heated together and are subjected to a common preliminary test by in each case one device for extraction, degassing and recording of the radioactivity, i.e. all the fuel assemblies belonging to the first group are tested together for leaks in the preliminary test, using the principle mentioned in the introduction, with the fuel assemblies remaining on the working base and continuous extraction of water, degassing and recording of the radioactivity taking place even during the heating. Although water is extracted, fission gases are therefore recorded and the testing takes place on-line, i.e. the first measured values for the radioactivity are already present and are being evaluated while extraction and degassing are still ongoing.
If the radioactivity in the gas which is liberated as a result of water being extracted from all the fuel assemblies belonging to the first group practically does not rise from the original measured level before the beginning of the test or the environmental level, the testing of the group is ended. Only in the case of a group which reveals significant radioactivity during the preliminary test are the fuel assemblies belonging to the group tested individuallyxe2x80x94but advantageously simultaneouslyxe2x80x94in which case a device for extraction, degassing and recording of the radioactivity is, of course, required for each fuel assembly belonging to the group (if the second group reveals significant radioactivity, therefore, in each case four devices for extraction, degassing and detection are usedxe2x80x94for example simultaneouslyxe2x80x94to individually test the fuel assemblies). Since the final testing of a significant group therefore requires a plurality of (four) devices of this type, devices provided for this purpose are also used for preliminary testing, preferably for the simultaneous preliminary testing of a plurality of groups (in the example, it is therefore possible for the first and second groups and two further groups belonging to the first division to be tested simultaneously).
However, a single group may also be left at the edge, and in a modification to the method this group is then considered to be its own division, and there may also be groups that have fewer fuel assemblies than the other groups (e.g. only a first fuel assembly). However, the invention allows even incomplete divisions and/or groups of this type to be tested using the same equipment.
In the most simple case, the fuel assemblies are heated in the manner described in the introduction, using the afterheat of the fuel assemblies. For this purpose, at least the fuel assemblies belonging to the first groupxe2x80x94and advantageously also all the other fuel assemblies belonging to the first divisionxe2x80x94are held under a common hood, in which, as a result of the introduction of gas, a gas cushion is generated above the fuel rods. The hood is only lifted off the fuel assemblies when the fuel assemblies have been tested.
Advantageously, the common hood is divided into cells above the fuel assemblies by side walls. In the most simple case, the way in which the hood is split into cells corresponds to the splitting of the division into groups.
To generate the gas cushion in each cell, it is preferable for gas to be passed into the cells until filling-level test lines that lead out of the cells indicate a predetermined height of the water level. Since not all the fuel assemblies are of the same height (for example because they originate from different manufacturers or because they have undergone different radiation-induced growth in the preceding operating cycles), but in the interests of sealing the fuel assemblies with respect to one another the fuel assembly channels are to extend all the way to the gas cushion as far as possible, the filling-level test lines in the individual cells can preferably be individually adjusted in terms of height in accordance with the height of the water level which has been predetermined for each cell. In this case, the filling-level test lines can be used simultaneously for venting, i.e. gas is introduced until gas escapes via the filling-level test lines. With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for testing fuel assemblies resting on a working base and under water, of a nuclear reactor. The apparatus contains a hood that can be positioned above at least one first fuel assembly and having a downwardly facing opening. The downwardly facing opening can be fitted over a top fitting of at least the first fuel assembly. A first device is connected to and introduces a gas into the hood. A second device is provided for continuously extracting water positioned under the gas introduced into the hood. A third device is connected to the second device and receives the water previously extracted, the third device continuously degasses the water resulting in a released gas. A fourth device is connected to the third device and receives the released gas for continuously recording a radioactivity of the released gas. A control device is connected to the second, third and fourth devices. The control device runs a program for controlling the second, third and fourth devices.
Accordingly, the device includes a hood which can be positioned at least over a first fuel assembly and a downwardly facing opening of which can be fitted over the top fitting of at least the one fuel assembly, and a device for introducing gas into the hood, and also devices for extracting a sample and for recording (detecting) the radioactivity of a gas. The device for extraction of a sample is suitable and intended for the continuous extraction of water under the gas that can be introduced into the hood. It is connected, by a device for the continuous degassing of water, to a following device for detecting the radioactivity of a gas, which can be continuously supplied with the gas released in the degassing device and is able to measure the radioactivity of gases continuously. Moreover, there is a control device for controlling the devices for extraction, degassing and recording of the radioactivity.
The device for recording the radioactivity of the gas is advantageously disposed in a disposal line that is connected to the degassing device. The gas that is produced continuously in the degassing device is continuously removed from the degassing device and then disposed of via the disposal line. Therefore, at least during the continuous extraction of the water, the degassing device and the device for recording the radioactivity of the gas are connected to one another only via the disposal line. This does not rule out the possibility of the degassing device and the device for recording radioactivity, before the fuel assembly is tested, being jointly connected to a venting and purging installation, but at least one suitable sequence control ensures that during the extraction of the water it is impossible for any extracted gas to be returned from the device for recording radioactivity to the degassing device. The above-mentioned venting and purging device is also advantageous for introducing a carrier gas into the extracted water in the degassing device, the carrier gas then providing gas bubbles and collection points for dissolved gaseous fission products in this extracted water. Any gaseous fission products that are driven out of the defective fuel rods are also degassed and, together with the carrier gas, are carried into the device for recording radioactivity, where they lead to an increase that at a very early stage makes it possible to reach a judgement about the state of the fuel rods.
Therefore, the apparatus, in a method according to the invention, in which, in a first division of fuel assemblies, which contains at least one first fuel assembly or a plurality of first fuel assemblies, at least one first fuel assembly is heated and in this way is tested, allows water to be continuously extracted as a sample even during the heating. The water is continuously degassed, and in the gas that is liberated the radioactivity of gaseous fission products released is continuously recorded. The heating and testing of a first fuel assembly belonging to a second division of fuel assembliesxe2x80x94in particular the fitting of a hoodxe2x80x94is only commenced when the first fuel assembly belonging to the first division has been tested. Advantageously, at least the first fuel assembly belonging to the first division is heated under a hood which has been fitted over a plurality or all of the fuel assemblies belonging to the first division and which contains a gas fill which surrounds the top fitting of the first fuel assembly(s) to be heated.
This avoids movement of the fuel assemblies; the number of time-consuming hood movements can be minimized, and it is possible to check for leaks from all the fuel assemblies belonging to one division in a common heating and preliminary testing step, which only has to be followed by the same number of individual testing steps on the fuel assemblies as the number of groups which are identified as significant. In the case of divisions containing at most four groups for in each case four fuel assemblies, in the least favorable situation, which scarcely ever occurs (i.e. that each of the four groups contains defective fuel rods), only five method steps are required.
This is possible because during the heating the fission products are driven out of the vicinity of a leak in a rod, and therefore the fission gases begin to escape as early as during the heating, but even in the fifth step fission gases are still being produced continuously from more remote locations in the fuel rod and in particular from pores which are close to the surface in the fuel rod. Although the surface temperature of the fuel rods asymptotically approaches a maximum temperature (e.g. 25xc2x0 to 40xc2x0 K. above the temperature of the reactor water, according to the, irradiation state and the afterheat of the fuel), the quantity of fission gas which constantly escapes and collects in the water of the fuel assembly is sufficient to unambiguously identify defective fuel assemblies even in the final step.
The on-line evaluation enables the test to be interrupted as soon as significant results are obtained. As a result, the time required to test a complete core, which has hitherto. generally been from 30 to 120 hours, is shortened to less than 15 hours.
A predetermined temperature difference (e.g. 10xc2x0 K. or less if significant measured values are already present) is sufficient for heating. Temperature monitoring is not required. For example, a fuel assembly can generally be considered intact and the test can be ended if the measurement results produced online reveal no increase in the radioactivity after a period of time which corresponds, for example, to heating by 10xc2x0 K. Only a very considerable scatter of measured values may make it necessary to wait for longer times to determine whether the radioactivity rises above the basic level by a predetermined minimum value. A reliable judgement in this respect can be reached at the latest after a heating time for which a value of between 10 and 25xc2x0 can be predetermined.
The measurement results can be evaluated intellectually or automatically. In particular, the heating and testing may be controlled automatically by a testing program that is started as soon as the hood has been positioned on the fuel assemblies and, for example, delivery pumps and valves are connected into the devices for extraction and detection.
Gas is preferably introduced under the hood until a filling-level test line that leads out of the hood indicates a predetermined height of the water level beneath the hood. In general, it is advantageous if the upper edge of the fuel assembly channel extends as far as the water level beneath the hood, since the interior of the fuel assembly channel is then insulated from extraction lines in other fuel assemblies. Therefore, fission product emerging from a defective fuel rod in one fuel assembly cannot enter an adjacent fuel assembly and invalidate the results for that assembly. The extraction of the water takes place as far as possible above the end plugs of the fuel rods (advantageously above the upper rod-holding plate in the top fitting of each fuel assembly), in order also to capture fission products that could escape at that location from leaking weld seams. This requires the positioning of the extraction lines and the water level beneath the gas cushion to take account of the individual length of the fuel assemblies.
The hood is preferably only fitted over at least one fuel assembly belonging to a different division, and the fuel assembly belonging to the other division is only inspected, when a check has been carried out for leaks from all the fuel assemblies belonging to the first division.
As has already been mentioned, the edge of the core grid can no longer be divided into grid meshes that form a complete group (four fuel assemblies). However, the apparatus and the method according to the invention can also be applied to these cases. In this case, therefore, the fuel assemblies belonging to one division, which in addition to a first or a plurality of first fuel assemblies also contains a second fuel assembly or a plurality of second fuel assemblies, are heated and tested. For this purpose, at least the fuel assemblies belonging to a first group, which includes at least one first fuel assembly and at least one second fuel assembly belonging to the first division, are heated together in a first step, and the fuel assemblies belonging to the first group are subjected to a common preliminary test, in which, during the heating, water is continuously extracted from each fuel assembly and is continuously degassed in a common degassing device. Moreover, by recording the radioactivity of the gaseous fission products contained in the gas that is released in the process, all the fuel assemblies belonging to the first group together are checked for leaks. Only in the event of significant radioactivity are, in a second step, the fuel assemblies belonging to the first group tested individually by independent, continuous extraction of water, independent degassing and independent recording of the radioactivity.
In the process, preferably the fuel assemblies which have been subjected to a common preliminary test in the first step are, in the second step, tested simultaneously, i.e. at the same time, but independentlyxe2x80x94therefore by in each case a dedicated extraction device, degassing device and detector device for each fuel assembly belonging to the group exhibiting the significant radioactivity.
Preferably, the cells or groups each contain the same number of fuel assemblies (i.e. four in the present example) and the number of devices for extraction is likewise equal to this number, in which case at least some of the extraction devices can be switched from the extraction lines of individual fuel assemblies belonging to one group to the extraction lines of individual fuel assemblies belonging to another group and to the combination of extraction lines belonging to a group. This enables extraction devices with the degassing and detector devices connected to them to be used a number of times, namely first to subject a plurality of groups to the preliminary test simultaneously and then to simultaneously subject the individual fuel assemblies belonging to a group which exhibits a significant radioactivity to the final test.
The result is that, in the conventional boiling water rector cores with a square pattern for the configuration of the fuel assemblies, it is particularly advantageous if the number of groups in the division is equal to the number of individual fuel assemblies in a group, i.e. if four groups each containing four fuel assemblies form a division which is tested beneath the hood.
In the context of the present invention, reactors whose fuel assemblies are hexagonal in cross section are treated in the same way as boiling water reactors, provided that they are surrounded by a channel (as is the case with some light water reactors constructed in Eastern Europe). In this case, it is advantageous for the divisions to be formed from two groups each containing three fuel assemblies or from three groups each containing two fuel assemblies. In this case and for the divisions and groups which occur at the periphery of the reactor core, in which it is impossible to fit every fuel assembly position beneath the hood, it is not possible to use all the devices which are present in every step, but rather some of them are shut down. It is also advantageously possible for extraction lines or filling-level lines at unoccupied positions of the hood at the core periphery to be blocked off.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and an apparatus for testing nuclear reactor fuel assemblies, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.